1. Field of the Disclosure
The present invention relates generally to the monitoring of variations in size of particles present in a fluid. More particularly, the present invention relates to methods and systems for monitoring variations in the size of particles present in a fluid by optical means.
2. Brief Description of Related Technology
Monitoring the size and amount of particles or contaminants present in a fluid is important in various industrial applications. This can be achieved by utilizing direct or indirect optical measurement methods, including scattering methods.
Scattering is a general physical phenomenon whereby some forms of radiation, such as light or moving particles, are forced to deviate from a straight path by one or more localized non-uniformities in the medium through which it passes. The types of non-uniformities that can cause scattering are sometimes known as scatterers or scattering centers. The scatterers may include particles, bubbles, droplets, density fluctuations in fluids, defects in crystalline solids, surface roughness, cells in organisms, and textile fibers in clothing etc. Scattering is a very general phenomenon which can take various forms depending on the size of the scatterer as compared to the wavelength of the radiation.
As known in art the scattering phenomenon can be categorized as Rayleigh scattering or Mie scattering. Rayleigh scattering involves scattering of light, or other electromagnetic radiation, by particles significantly smaller than the wavelength of the light. For particle sizes comparable to or somewhat larger than the interrogating wavelength, Mie scattering predominates. At values of the ratio of particle diameter to wavelength more than about 10, the laws of geometric optics are mostly sufficient to describe the interaction of light with the particle, and at this point the interaction is not usually described as scattering.
Known optical methods for different applications use various wavelengths including wavelengths that are smaller, about the same as or larger than the size of the particles to be measured. One such application is to monitor the amount and size of soot particles in engine lubricating oil (hereafter referred to as ‘engine oil’) wherein the intensity of the interrogating optical signal when passed through the engine oil containing non-uniformities/particulate material can be analyzed to monitor the condition of the engine oil.
In standard internal combustion engines, most of the soot generated by the combustion of the fuel is released to the atmosphere via the exhaust gas, while a small fraction entered the crankcase and the engine lubricant via combustion gas blow-by (i.e. combustion gas leaking pass the piston rings, valve guides, etc.), or through deposition on cylinder walls that was subsequently scraped off by the piston rings and ultimately deposited into the lubricating oil. Further, due to more rigorous nitro-oxidation and hydrocarbon emission policies of the environmental protection agencies, engine manufacturers are increasingly using exhaust gas recirculation (EGR) to reduce atmospheric soot emissions. EGR sends some of the engine emissions, including soot, back to the combustion chamber creating a multi-pass opportunity for the soot to ingress to the lubricating oil and thereby increasing the rate of growth of soot concentration in the lubricant.
For most combustion engines, the soot content (or soot load or soot weight) increases more or less linearly with the number of hours of operation since the last oil change. As time progress the amount of soot trapped in the engine oil can reach significant levels, especially in Diesel engines. Under some conditions the accumulation of soot in the engine oil can prevent proper lubrication of the engine parts and can significantly accelerate engine wear. It is therefore important to be able to monitor the presence of soot and/or other contaminations in internal combustion engines and/or any other device where such particles may be present. Soot particles comprise the largest volume of contaminants present in used diesel engine lubrication oil. When large amount of soot is present, the lubricating oil viscosity is altered, and the oil may no longer be able to properly lubricate the engine. Furthermore large agglomerated soot particles cause abrasive action on the engine bearings. This is most insidious in regions of high-load where oil films are on the order of microns or less.
In fresh lubricating oil, the soot particles are suspended in the oil, surrounded by additives such as dispersants whose function is to prevent soot particle growth. When the dispersant is consumed or prevented to perform its function, the small soot particles can aggregate to form much larger elements. The risk to the engine is limited as long as these small soot particles remain suspended in the oil and are not allowed to agglomerate to form larger particles. However, when the soot particles grow to be of a size similar to that of the lubricating oil film thickness as found on critical parts, significant wear to the engine may rapidly occur. This condition is highly undesirable and there has been a need for systems to monitor the condition of the engine oil so that damages associated with the wear due to contaminated engine oil can be avoided or minimized.
Some of the prior art systems monitor condition of the engine oil by indirect measurements, for example by measuring surrogates, such as dielectric constant. However, these surrogates may be influenced by many parameters besides soot such as moisture, ethylene glycol, etc. It is difficult to assess the state of the engine oil using these indirect measurements.
The U.S. Pat. No. 5,309,213 titled, “Optical determination of amount of soot in oil sample” assigned to Desjardins, John B. et al., proposes a method and apparatus for determining the concentration of materials in fluids, such as soot in diesel engine lubricating oils. An optical cell is employed in the form of a very thin tapered sample volume which is inserted into a light beam. The attenuation of the beam is measured at different locations along the varying thickness of the sample. The invention of Desjardins, et al is focused towards an improved optical cell for determining soot concentration in engine oil. Desjardin's monitor the optical transmission at one point in time. This approach does not provide information about the soot particles physical size, or the soot particle growth. Furthermore Desjardin's approach requires movement of the analytical cell in order to perform the measurement.
There is therefore a need for a method and system for a real-time and in-field monitoring the condition of soot particles present in the engine oil in order to minimize potentially high-risk circumstances of wear and damage of the engine.